Layout 1 ARTICLE Introduction Lake Toba is located in North Sumatra Province, and is the largest lake in Southeast Asia. The surface area, water volume, and maximum depth of Lake Toba are 1,124 km2, 256.2×109 m3, and around 508 m, respectively (Garno et al., 2020). Lake Toba is surrounded by seven regencies, i.e., North Tapanuli, Simalun- gun, Dairi, Humbang Hasundutan, Karo, Samosir, and Toba Samosir. Lake Toba provides many ecological and economic benefits. Ecologically, Lake Toba is a habitat for various en- demic and non-endemic freshwater organisms. Anggreini and Supriyadi (2019) documented the presence of several fish species in Lake Toba, including Batak fish (Tor douronensis Va- lenciennes), Mozambique tilapia (Oreochromis mossambicus Peters), Nile tilapia (Oreochromis niloticus Linnaeus), marble goby (Oxyeleotris marmorata Bleeker), cork (Channa striata Bloch), goldfish (Cyprinus carpio Linnaeus), bilih (Mystacoleu- cus padangensis Bleeker), Java barb (Barbonymus gonionotus Analysis of main components of Lake Toba’s water quality in different seasons Yuni Puji Hastuti1, Kukuh Nirmala1, Manuntun Parulian Hutagaol2, Dahri Tanjung3, Agit Kriswantriyono3, Wildan Nurussalam1, Yulia Puspadewi Wulandari3, Yuli Siti Fatma4 1Department of Aquaculture, Faculty of Fisheries and Marine Science, IPB University, Bogor; 2Department of Economics, Faculty of Economic and Management, IPB University, Bogor; 3Center for Alternative Dispute Resolution and Empowerment (CARE LPPM IPB), IPB University, Bogor; 4Research Center for Applied Microbiology, Research Organization for Life Sciences and Environment, National Research and Innovation Agency (BRIN), Bogor, Indonesia ABSTRACT Lake Toba is one of the largest lakes in North Sumatra Province, Indonesia. Its waters are used for multiple purposes, constituting an important natural and economic resource. Most of the waters of Lake Toba come from the overflow of disposal of agriculture, livestock, fisheries, tourism, households, and other activities. The present study identified water quality based on total nitrogen, total phosphorus, chlorophyll-a and other water quality parameters carried out at 60 sampling sites grouped into 6 obser- vation stations, i.e., control areas (located in the middle and far from direct activities), community floating net cages, company floating net cages, settlement, hospitality, and river mouths. The main water quality components were very dynamic at the moni- toring stations in three seasons (rainy, transition, and dry). Total nitrogen concentration tended to be higher in the rainy season than in the transition and dry seasons. However, all stations tended to be classified as mesotrophic or higher in all seasons, with total ni- trogen concentrations greater than 12.5 mg L–1. Total phosphorus at the six stations was highly dynamic in all seasons and tended to decrease in the dry season. The concentration of total phospho- rus was higher at the settlement and hospitality stations than at the other stations. The total phosphorus of the settlement and hospi- tality stations reached 0.18 mg L–1 and 0.17 mg L–1, respectively, in the rainy season. In general, total phosphorus concentrations in the waters of Lake Toba were above 0.1 mg L–1, which allowed the lake to be classified as above mesotrophic status. Corresponding author: Yuni Puji Hastuti, Department of Aquacul- ture, Faculty of Fisheries and Marine Science, IPB University, Jl. Raya Dramaga, Bogor 16680, West Java, Indonesia. Tel.: +62 251 8622909-8622911; Fax: +62 251 8622907. E-mail: yuniha@apps.ipb.ac.id Key words: environmental quality, freshwater, Lake Toba, seasons, total nitrogen, total phosphorus. Authors’ contributions: YPH and KN contributed to the conception and design of the experiment, the analysis and the interpretation of data. All the authors wrote the article draft and revised it criti- cally for important intellectual content. All the authors have read and approved the final version of the manuscript and agreed to be held accountable for all aspects of the work. Conflict of interest: the authors have no conflicts of interest to de- clare. Funding: the Indonesia Endowment Fund for Education (LPDP) of the Ministry of Finance, Republic of Indonesia, funded this work. Availability of data and materials: all data generated or analyzed during this study are included in this published article. Acknowledgments: the authors thank the Indonesia Endowment Fund for Education (LPDP) of the Ministry of Finance, Republic of Indonesia, for financial support; the Center for Alternative Dis- pute Resolution and Empowerment (CARE LPPM IPB), IPB Uni- versity and the Marine and Fisheries Agency of North Sumatra Province for supporting this research. Received: 8 September 2023. Accepted: 21 December 2023. Publisher’s note: all claims expressed in this article are solely those of the authors and do not necessarily represent those of their affil- iated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher. ©Copyright: the Author(s), 2024 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2024; 15:11726 DOI: 10.4081/aiol.2024.11726 This work is licensed under a Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). Non -co mmerc ial us e o nly Y.P. Hastuti et al.2 Bleeker), and betok fish (Anabas testudineus Bloch). Econom- ically, Lake Toba is currently used as a water source for people’s daily needs, such as drinking water and clean water. In addition to being a tourist destination, Lake Toba is also an important site for floating net cages for fish farming and marketing. Currently, Lake Toba is used for community livelihoods (household needs, including drinking water sources) and other in- dustrial activities (tourism, agriculture, fisheries, and livestock). Human activities, such as fishing and fish farming in floating net cages, settlements, agriculture, livestock, and tourism, can be a source of pollution in Lake Toba. Some reports indicate that the current status of Lake Toba waters is getting worse every year (Lukman et al., 2021; Barus et al., 2022). Since 2009, the lake shifted from oligotrophy to mesotrophy (Barus et al., 2022). It would be difficult to avoid this condition because most human ac- tivities surrounding Lake Toba originate from and drain into the lake. Consequently, the capacity of the water body is reduced due to the high concentration of agricultural fertilizers dissolved in water, domestic waste discharged into the lake, feed and fecal residues from fish farming on floating net cages, and other excess organic waste. Excessive organic waste from floating net cages stimulates the decomposition process, leading to a decrease in dis- solved oxygen and an increase in H2S, ammonia, nitrogen, and phosphorus compounds (Garno et al., 2020). The various development activities in the Lake Toba area, which are increasing rapidly, require various studies on the rela- tionship between nutrient conditions and water inputs. Water qual- ity analysis, including physical, chemical, and biological parameters, continues to be carried out to determine the current status of the Lake Toba environment. Not only physical and chem- ical variables are monitored, but biological variables are also as- sessed to determine water quality. Phytoplankton is one of the aquatic organisms playing a key role as primary producers in the water. Phytoplankton respond to changes in water conditions with changes in abundance, species number, and community structure (Ferreira et al., 2011). The abundance of phytoplankton is influ- enced by several factors, including nutrients, light conditions, tem- perature, pH, and predation by zooplankton and planktivorous fish (Lau and Lane, 2002; Jiang et al., 2014). In turn, these factors are also strongly influenced by the seasonal climate. Based on the above considerations, the main objective of this work was to assess the current trophic status of Lake Toba in three different climatic seasons, i.e., rainy, transition, and dry seasons. The main sources of pollution affecting the trophic level of Lake Toba were identified by measuring nutrients and phy- toplankton biomass (UNEP-ILEP, 1990). Several points are con- sidered as water sources or suppliers relevant for the study, i.e., control areas, settlements, hospitality, community floating net cages, company floating net cages, and the river mouth. There- fore, another important objective of this work was to assess the potential conflicts arising from the use of water resources for multiple purposes. Materials and Methods Description of the study area This research was carried out from 2020 to 2021 in Lake Toba, North Sumatra Province, Indonesia, during the three main seasons defined by the regional weather forecast information by the Meteorology, Climatology and Geophysics Agency (BMKG; available from http://www.bmkg.go.id) (rainy, transition, and dry). Samples were collected in February 2020 (rainy season), March 2021 (transition season), and August 2021 (dry season). A total of 2 L of water was collected at 60 sites representing 6 observation stations, i.e., control (sites away from various human activities), settlements, hospitality (hotels and restau- rants), community floating net cages, company floating net cages, and river mouths (Supplementary Figure 1; Supplemen- tary Table 1). The identification of the six categories is based on the assessment of activities that mainly affect the water entering Lake Toba. Samples from each station were taken from 3 inte- grated water layers, i.e., 0-1 m, 2-3 m, and 3-6 m (Supplemen- tary Figure 1). The measurement of water quality The variables measured in this study included water temperature (°C), Dissolved Oxygen (DO, mg L–1), Total Nitrogen (TN, mg L–1), Total Organic Matter (TOM, mg KMnO4 L–1), Total Phos- phorus (TP, mg L–1), chlorophyll-a (APHA, 2005), and bright- ness and water transparency. TOM measurement was carried out according to SNI 06- 6968.22-2004 (Indonesian National Standard) with the follow- ing formula: (1) where: TOM: Total Organic Matters (mg L–1) X: titrant volume for water sample (mL) Y: titrant volume for aquadest (blank solution) (mL) 31.6: 1/5 from molecular weight of KMNO4 0.01: normality of KMNO4 V: sample volume (mL) Brightness was measured following (Carlson, 1977): (2) where: TSI: Tropic State Index SD: Secchi Disk depth (m) TSI and chlorophyll-a concentration were measured follow- ing (Carlson, 1977): (3) where: ChlA: Chlorophyll-a concentration (mg m–3) (Simon and Hel- liwell, 1998) TSI and total phosphorus were measured following (Carl- son, 1977): (4) where: TP: Total Phosphorus concentration (mg m–3) Kratzer and Brezonik (1981) developed Carlson’s TSI equa- tion for total nitrogen as follows: (5) where: TN: Total Nitrogen concentration (mg L–1) Non -co mmerc ial us e o nly Lake Toba water quality in different seasons 3 Data analysis Data were analyzed using Pearson correlation, simple linear regression, and dendrogram using Minitab 18 software. Sam- pling stations were grouped based on the similarity of the water quality using the average values assessed by computing cluster (Canberra distance) and biplot analyses (PCA) using the R 4.1.2 software. The cluster plot and biplot analyses used one repre- sentative unit per station for a total of six stations. Results Water brightness showed significant differences between sampling stations in each season (Table 1; Supplementary Figure 2). In the rainy season, the sampling sites with the highest bright- ness value were located in the control zones and company float- ing net cages (5.31 m and 5.62 m, respectively). The higher the brightness value in water. the higher the penetrating light and the lower the organic matter or dissolved particles. The lowest bright- ness values in each season were found in the hospitality sites, with an average value between 2.08 and 2.92 m (Table 1; Sup- plementary Figure 2). Excluding the hospitality sites, these values were still within the standard limits set by Minister of Environ- ment Decree No. 51/2004, which states that the brightness level of the waters should be >3 m. In the hospitality sites, water brightness values were in agreement with TOM values, which tended to be stable at a high level in the dry season (see below). The water temperature was higher in the transition season. The highest temperature, 27.32°C, was found in community floating net cages (Supplementary Figure 3). The lowest DO was measured at the control station in the transition season (Supplementary Figure 4). Low DO values of <5 mg L–1 were found in each season at the control, hospitality, and settlement stations. The highest DO value of >5 mg L–1 in each season was consistently found in the company’s floating net cage station (Supplementary Figure 4). The highest level of total nitrogen decreased from the rainy season to the dry season (Supplementary Figure 5). The control station had the highest total nitrogen value, reaching 24.2 mg L–1 in the rainy season, while the settlement station had the greatest total nitrogen value of 18.0 mg L–1 in the dry season. High variability of total phosphorus concentrations was ob- served in several stations of Lake Toba (Table 1; Supplementary Figure 6). The highest values in the rainy, transition, and dry season were measured in the settlement (0.18 mg L–1), control (0.17 mg L–1), and hospitality stations (0.16 mg L–1), respec- tively (Table 1; Supplementary Figure 6). In the rainy season, TOM concentration had relatively sim- ilar values at each season and station, ranging from 130.4 to 133.9 mg KMnO4 L–1 (Supplementary Figure 7). Each station showed a decrease in the TOM concentrations in the dry season. The lowest and the highest TOM values were measured in the control station (87.7 mg KMnO4 L–1) and in the settlement sta- tion (125.8 mg KMnO4 L–1) (Supplementary Figure 7). Chlorophyll-a concentration showed higher variability be- tween stations in the transition season (Table 1; Supplementary Figure 8). In this season, the smallest and largest concentrations were measured in the control station and the community floating net cages, respectively. The first two dimensions of PCA described the 79.7% Ta bl e 1. T he ra ng e an d av er ag e va lu e of th e Se cc hi D is k de pt h (m ), to ta l p ho sp ho ru s ( m g L–1 ) a nd c hl or op hy ll- a (μ g L–1 ) i n ea ch st at io n du rin g ra in y, tr an si tio n, a nd d ry se as on s. Sa m pl in g si te s R ai ny T ra ns iti on D ry S ec ch i d is k T ot al C hl or op hy ll- a S ec ch i d is k T ot al C hl or op hy ll- a Se cc hi d is k To ta l C hl or op hy ll- a d ep th (m ) p ho sp ho ru s ( µg L –1 ) de pt h (m ) p ho sp ho ru s ( µg L –1 ) de pt h (m ) p ho sp ho ru s ( µg L –1 ) (m g L –1 ) (m g L –1 ) (m g L –1 ) R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge R an ge A ve ra ge C on tro l 4 .0 0- 6. 50 0 5 .3 1 0. 08 -0 .2 7 0. 14 0 .0 0- 4. 47 2 .5 5 0. 30 -5 .5 0 3. 20 0 .0 4- 0. 96 0 .1 7 1. 66 -5 .2 3 3. 09 2 .0 0- 5. 50 3 .8 1 0. 06 -0 .1 3 0. 08 1 .6 4- 4. 82 3 .3 8 Se ttl em en ts 0. 33 -6 .0 0 4. 10 0 .1 2- 0. 42 0 .1 8 1. 33 -4 .4 2 2. 70 1 .0 0- 4. 50 2 .9 3 0. 04 -0 .3 0 0. 09 2. 23 -1 6. 65 5. 87 0 .5 0- 5. 00 3 .4 2 0. 06 -0 .3 8 0. 13 0 .8 6- 5. 81 3 .6 0 H os pi ta lit y 0 .2 00 -4 .0 0 2 .1 2 0. 09 -0 .2 8 0. 17 1 .4 4- 3. 84 3 .0 7 0. 50 -3 .5 0 2. 08 0 .0 4- 0. 14 0 .0 8 2 .4 1- 12 .2 0 6 .0 1 1. 00 -4 .5 0 2. 92 0 .0 6- 0. 41 0 .1 6 2. 88 -5 .8 4 4. 19 C om m un ity fl oa tin g 0. 60 -5 .5 0 3. 48 0 .0 7- 0. 27 0 .1 1 1. 59 -6 .4 5 4. 20 2 .5 0- 5. 00 3 .5 7 0. 04 -0 .0 6 0. 05 2. 46 -2 2. 24 7. 92 1 .5 0- 6. 50 3 .1 5 0. 05 -0 .1 2 0. 07 2 .0 6- 4. 66 3 .4 7 ne t c ag es C om pa ny fl oa tin g 4 .5 0- 6. 30 5 .6 2 0. 08 -0 .1 4 0. 11 3 .4 0- 6. 15 4 .6 8 3. 40 -5 .0 0 3. 95 0 .0 4- 0. 07 0 .0 6 2. 36 -4 .3 2 3. 31 3 .0 0- 5. 00 3 .9 5 0. 06 -0 .1 6 0. 10 2 .6 5- 4. 09 3 .6 6 ne t c ag es R iv er m ou th s 0 .5 0- 6. 50 3 .8 3 0. 07 -0 .2 7 0. 14 1 .4 4- 7. 00 3 .4 3 1. 00 -5 .5 0 3. 26 0 .0 4- 0. 14 0 .0 7 1 .4 4- 16 .6 5 4 .8 2 0. 50 -5 .5 0 3. 04 0 .0 5- 0. 18 0 .0 9 1. 61 -7 .2 1 3. 77 Non -co mmerc ial us e o nly Y.P. Hastuti et al.4 (PCA1, 47.6%; PCA2, 32.1%) of the total variance (Supplemen- tary Figure 9). The biplot analysis showed a negative association between TOM and brightness, DO, temperature and, partly, total nitrogen. Also, TOM was positively associated with chlorophyll- a and TP. Hospitality sites showed a higher influence of TOM, TP and, partly, chlorophyll-a. Community floating net cages and river mouths were mostly associated to total nitrogen and tem- perature. Company floating net cages and control stations were affected by DO and brightness. The settlement station was as- sociated to TP and, partly, TOM. The observation stations were grouped into two clusters (Supplementary Figure 10), including the control and the com- pany floating net cages stations, and the hospitality, settlement, and river mouths stations, along with, at a higher distance, the community floating net cages. The stations with the highest sim- ilarity in water quality characteristics were the settlement and river mouths. Discussion The characteristics of the waters of Lake Toba were very dy- namic from time to time, changing from one station to another. The variability can be caused by the high waves that even reach 2.5 m in each season, which stirs the water body and affects the movement of the water. Generally, the highest brightness values were found at the company’s floating net cages during the rainy season, while the lowest brightness values were found at the hospitality stations during the transition season (Table 1; Supplementary Figure 2). The high brightness level in the company floating net cages might be due to the proper selection of the sites, which are char- acterized by high depths, and to the large amount of organic and inorganic compounds that are transported by water currents to other stations. In contrast, the low level of brightness at the river mouth was presumably due to the carrying of particles into the water flow (runoff) resulting from anthropogenic activities, such as agriculture, animal husbandry, households, and others, from land to the lake. The highest temperature was observed at the community floating net cages sites during the transition season, while the lowest temperature was observed at the settlement sites during the transition season (Supplementary Figure 3). The water tem- perature of the six stations remained within the acceptable levels according to the Regulation of the Minister of Health (Perme- nkes), Republic of Indonesia, No. 32/2017 concerning the envi- ronmental health quality standards and water health requirements for the needs of sanitary hygiene, swimming pool, solus per aqua (health through water), and public baths. The low temperature in a water body is a consequence of the lack of sun- light penetration (Parker, 2012). Solar radiation, air temperature, weather, and climate are several factors that affect water tem- perature distribution (Boyd, 2015). The low temperature de- tected at the settlement, reaching 24.73°C in dry seasons, could be affected by the high turbidity caused by organic and inorganic particles (Supplementary Figure 3). In the dry season, the inflow of water from settlement, hospitality, agriculture and all water sources into the lake directly affects its turbidity. During the dry season, there will be an increase in dissolved particles in the water, which will inhibit water productivity and increase the tur- bidity level. In addition, an increase in organic and inorganic particles in the water can limit the penetration of sunlight and cause uneven photosynthetic processes in the water, especially in deep areas. This result was in line with the TOM concentra- tions, which were stable at a high level in the settlement (Sup- plementary Figure 7). Low temperatures can constrain the metabolism of aquatic biota. Activity in some fish populations decreases at temperatures <20°C (Parker, 2012), and the ideal temperature for the Tilapia group is between 26.1 and 32.2°C or between 26 and 30°C (De et al., 2016). The highest DO concentrations were measured in the com- pany’s floating net cages during the dry season, while the smallest DO concentrations were measured at the control stations in the transition season (Supplementary Figure 4). Similar to the bright- ness level, the high oxygen levels in the company floating net cages are thought to be due to the proper selection of the sampling sites, taking into account the depth and underwater currents so that the oxygen level tends to be higher than the other stations. Further, high DO levels could be affected by the high brightness level, leading to a high light penetration and photosynthesis ac- tivity by phytoplankton. In addition, the intensity of sunlight dur- ing the dry season is high, which promotes the photosynthetic process of plankton in the waters around the company’s floating net cages and increases the DO concentration (Garno et al., 2020). Conversely, the low DO concentrations might be caused by the high turbidity level because of dissolved organic matter or sus- pended particles which directly and indirectly affect the level of brightness and DO. The DO concentrations in Lake Toba are still within the tolerance range for aquatic biota according to the stan- dard quality of >5 mg L–1 set by the Indonesian government on the Decree of the State Minister of Environment No.51/2004. The average total nitrogen in Lake Toba waters ranged from 12.5 to 26.9 mg L–1 (Supplementary Figure 5). According to the Decree of the State Minister of Environment No.42, the thresh- old concentration of ammonia (NH3-N) in water is 5 mg L–1. Meanwhile, Governor’s Decree No. 45/2002 set that the maxi- mum nitrite (NO2-N) level in water is 3 mg L–1. Further, accord- ing to the Governor’s Decree No. 45/2002, the quality standard for water’s nitrate (NO3-N) level is not more than 30 mg L–1. Davidson et al., (2015) reported that the high levels of nitrogen contamination in open water areas used by the community can have adverse effects on human health when consumed as drink- ing water. Wurtsbaugh et al., (2019) described that the high ni- trogen level in water is able to contribute to the harmful algae blooms threatening the inhabiting organisms. Total nitrogen is one of the test parameters determining the trophic status of lake waters (UNEP-ILEP, 1990). In the present study, the total nitro- gen concentrations indicated a trophic status between eutrophy and hypertrophy with values >1.9 mg L–1 and a mean value of total nitrogen of >12.5 mg L–1 in all stations (KLH, 2009; OECD, 1982; MAB, 1989; UNEP-ILEC, 2001). In Lake Toba, TP concentrations were relatively high, with an average range of 0.048-0.177 mg L–1 in all observation sta- tions (Table 1; Supplementary Figure 6). The high concentration of TP in the settlement during the rainy season was presumably due to the large amount of household waste containing deter- gents, agricultural activities using fertilizers, household activities residues, and fish or livestock feed leftovers leaching into water bodies. These factors are phosphorus sources promoting fertility in waters. These findings are in line with the study performed Non -co mmerc ial us e o nly Lake Toba water quality in different seasons 5 by Handayani et al., (2011), who reported that the highest phos- phate concentration was found in the settlements in Lake Batur, Bangli Regency, Bali Province. According to Carlson (1977), lakes with mean TP concentrations greater than 0.1 mg L–1 are classified as eutrophic. Meanwhile, other reports stated that lakes with TP concentrations >0.1 mg L–1 are categorized as hy- pertrophic (KLH, 2009; OECD, 1982; MAB, 1989; UNEP- ILEC, 2001). Therefore, based on the range of TP of 0.005-0.02 mg L–1, the trophic status of Lake Toba was between eutrophy and hypereutrophy. The settlement and hospitality stations had hypertrophic fertility levels in the rainy and dry seasons. In the transition season, hypertrophic fertility was found in the control stations. According to Government Regulation No.82/2001 on management of water quality and control over water pollution, TP concentrations in Lake Toba remained within the acceptable level for public consumption, the maximum acceptable concen- tration of 0.2 mg L–1. The average concentration of TOM at the six observation stations ranged from 87.7 to 162.7 mg KMnO4 L–1 (Supplemen- tary Figure 7). The concentration of TOM in the Lake Toba wa- ters was very high. The TOM levels were above the findings reported by Lukman et al., (2013), who observed that the highest TOM content in Lake Maninjau was 25 mg KMnO4 L–1. The high TOM in the settlement and hospitality stations might have resulted from river mouth activities around the lake. Organic waste that can pollute the lake comes from the floating net cages and various activities in the surrounding area. Haryadi (2003) also reported that organic wastes entering public waters come from food, excreta, detergents, cleaning agents, oils and fats, suspended materials, residual insecticides, pesticides, and other synthetic materials. Organic wastes, consisting of carbon, hy- drogen, oxygen, nitrogen, phosphorus, sulfur, and other elements and minerals entering public waters come from various human activities, including household-scale industry, settlement, live- stock, agriculture, and fisheries (Porpraset, 1989). In general, the TOM concentration of Lake Toba exceeded the quality stan- dards set by the Decree of the Minister of State for Population and Environment No. 2/1988 which states that the TOM thresh- old is 80 mg KMnO4 L–1. Organic wastes are used by het- erotrophic microorganisms as energy and carbon sources for their growth resulting in a lack of oxygen in water. The abun- dance of nutrients triggers the acceleration of the eutrophication process in the reservoir waters. For example, nitrogen and phos- phorus in the Cirata reservoir (Indonesia) come predominantly from domestic waste, accounting for 2,111 tons of nitrogen waste per year and 276.6 tons of phosphorus waste per year, while the fishery activity of floating net cages contributes by 6,612 tons of nitrogen waste per year and 1,041 tons phosphorus waste per year (Brahmana and Ahmad, 1997; Garno et al., 2002). Likewise, for the Saguling reservoir, the highest annual discharge of nitrogen and phosphorus waste come from house- hold or residential waste (Garno et al., 2002). Trophic and nutrient status greatly influence macro- and mi- croorganisms structure and functionality in water ecosystems. Ions, minerals, inorganic-organic materials, and toxic pollutants originating from point and non-point sources (Davis and Corn- well (1991) strongly affect the survival, diversity, and abundance of macro- and microorganisms. Nutrient levels control the con- centrations of chlorophyll-a (a proxy for phytoplankton bio- mass). In turn, chlorophyll-a content influences light absorption and water transparency. The highest concentration of chloro- phyll-a was detected at the community floating net cages during the transition season, and the lowest concentration was at the control stations during the rainy season (Table 1; Supplementary Figure 8). Therefore, the community floating net cages at that time had the highest concentration of phytoplankton. High phy- toplankton concentrations can increase the DO concentration in lakes (Hikmawati et al., 2014). Generally, the chlorophyll-a con- centrations in Lake Toba were between 2-6 µg L–1, which can be included in the eutrophic range (Hakanson & Bryhn, 2008; KLH, 2009; OECD, 1982; MAB, 1989; UNEP-ILEC, 2001). The increase in TOM concentrations in Lake Toba was as- sociated with a decrease in the brightness and DO (Supplemen- tary Figure 9). In addition, the increase in TOM in the water was accompanied by an increase in chlorophyll-a and TP concentra- tions (Supplementary Figure 9). The settlement and river mouths had the highest similarity in water quality characteristics (Sup- plementary Figure 10). This result might be due to waste from the other settlements and surrounding areas affecting the water quality from the river mouth. Pollutants decrease the water qual- ity of Lake Toba, while enhancing water column anoxia in the hypolimnion layer and eutrophication as observed in November 2017 (Lukman et al., 2021). The physical, chemical, and bio- logical properties of Lake Toba’s water quality are heavily in- fluenced by the numerous pollution sources. The water quality of Lake Toba is altered by the water quality of river water, domestic public waters, agriculture, industry, live- stock, floating net cage farming activities, and other activities around and in the lake. A previous study reported some parties in- terested in the exploitation of Lake Toba area resources with the potential to cause pollution, including tourism actors, floating net cages actors, local communities, farmers, pig breeders, local gov- ernment, lake transport companies, industrial forests, mineral water companies, and non-governmental organizations. These nu- merous interests have the potential to raise conflicts in the multiple uses of water resources, principally between tourism and other parties (Tanjung and Hutagaol 2019). Three were the dominant activities affecting the Lake Toba area, i.e. tourism, floating net cages, and non-governmental organizations, with the potential for serious conflicts (Tanjung and Hutagaol 2019). Domestic waste has been reported as the main source of water quality degradation in Lake Toba. In addition, floating net cages in the aquaculture sector in Lake Toba are one of the main sources of pollution (Tanjung and Hutagaol 2019). The Gover- nor of North Sumatra has issued two decrees, No.188.44/ 213/KPTS/2017, which set the water quality of Lake Toba as oligotrophic and the maximum carrying capacity of the com- pany’s floating net cage at 10,000 tons/year. Tanjung et al. (2022) proposed that the carrying capacity for floating net cages should be revised from 10,000 tons per year to 67,000 tons per year in relation to the community’s economy. In the tourism sec- tor, Tanjung and Hutagaol (2019) reported the potential conflicts in the Lake Toba region related to the international ecotourism development, such as resort, restaurant, and hotels, which in- cludes the floating net cage company actors with tourism actors; floating net cage industry actors with the government; commu- nity floating net cage with company floating net cage; local com- munity with the government; and among communities. Tanjung and Hutagaol (2019) recommended that these conflicts could be addressed by involving all stakeholders in policy making, revis- Non -co mmerc ial us e o nly Y.P. Hastuti et al.6 ing the regulation of the Governor, strengthening the floating net cage owner group, and developing eco-edu-tourism. Co- management is needed between the government and the other parties that use Lake Toba’s water to ensure its sustainability. Water quality analysis during three seasons (dry, transition, and rainy) provides information on variations in water quality that can affect all activities and the carrying capacity of the lake. Specific sites had lower dissolved oxygen levels compared to other locations (Supplementary Figure 4). High levels of dis- solved substances in the environment can cause low oxygen lev- els. Hospitality and settlement with the highest nitrogen and phosphorous levels were associated with low oxygen and high organic matter, impairing the overall physical, chemical, and bi- ological quality of the water. Comprehensive wastewater man- agement is urgently needed in the future, especially for all sources potentially contaminating Lake Toba water. Conclusions According to the observations carried out in six station groups, including control (far from human activities), settlement, hospitality, community floating net cages, community floating net cages, and river mouths, the main water quality variables in Lake Toba showed a strong difference in the rainy, transition, and dry seasons. The waters of Lake Toba during these seasons tend to be classified above mesotrophy. Management of Lake Toba ecosystems should be carried out regularly to monitor en- vironmental quality and functionality, which are essential ele- ments to support and maintain ecosystem services. References American Public Health Association (APHA). 2005. Standard methods for the examination of water and waste water (21th ed.). American Public Health Association; Washington, USA. Anggreini DP, Supriyadi F. 2019. Estimasi standing stock sumber daya ikan di Danau Toba, Sumatera Utara. Sainmatika: Jurnal Ilmiah Matematika dan Ilmu Pengetahuan Alam. 16:176-84. 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Determination of trophic status of the Lake Toba according to water quality parameters. Supplementary Figure 1. Sixty sampling sites representing six stations in Lake Toba. Supplementary Figure 2. Water brightness at six Lake Toba observation stations in each climatic season. Supplementary Figure 3. Water temperature at six Lake Toba observation stations in each climatic season. Supplementary Figure 4. Dissolved oxygen at six Lake Toba observation stations in each climatic season. Supplementary Figure 5. Total nitrogen at six Lake Toba observation stations in each climatic season. Supplementary Figure 6. Total phosphorus concentrations at six Lake Toba observation stations in each climatic season. Supplementary Figure 7. Total organic matter (TOM) at six Lake Toba observation stations in each climatic season. Supplementary Figure 8. Chlorophyll-a concentrations at six Lake Toba observation stations in each climatic season. Supplementary Figure 9. Principal component analysis of water quality variables recorded at the six observation stations. Supplementary Figure 10. Cluster dendogram of six observation stations according to water quality variables using Canberra distance. Non -co mmerc ial us e o nly